As they skate or snack in Bryant Park, visitors might dismiss the stately New York Public Library next door as a dog-eared relic in an age of digital information.

But unbeknown to most of them, 17 feet below ground, in a concrete bunker worthy of the White House, the library is expanding and updating one of the most sophisticated book storage systems in the world.

Since March, after abandoning a much-criticized plan to move the bulk of its research collection to New Jersey, the library has been working instead to create a high-tech space underground for the 2.5 million research works long held in its original stacks.

The books will begin arriving in April, and by the end of spring library officials expect to be using a new retrieval system to ferry the volumes and other materials from their 84 miles of subterranean shelving, loaded into little motorized carts — a bit like miniaturized minecars carrying nuggets of research gold.

To fit all the books in the allotted space, the library will have to abandon its version of the Dewey Decimal System, in which shelving is organized by subject, in favor of a new “high-density” protocol in which all that matters is size.

Books will be stacked by height and tracked by bar code rather than by a subject-based system, making for some odd bookfellows. “Best Food Writing 2013,” edited by Holly Hughes, for example, will be stored next to “War Dog: The No-Man’s Land Puppy Who Took to the Skies,” by Damien Lewis; and “Old Romanian Fairy Tales,” translated by Mirela Roznoveanu, will sit beside “The Hot Dog Companion: A Connoisseur’s Guide to the Food We Love,” by David Graulich.

Librarians nationwide are embracing size-based systems as they retool their research collections, which unlike books that circulate, cannot leave the premises or be browsed by hand.

“It’s a lot better,” said Carolyn Broomhead, the library’s research community manager. “Things don’t get squished together and are much easier to find and track.”

The Bryant storage space consists of two floors, the first of which was put into use in the late 1980s, while the second floor, dug out but not finished back then, has lain fallow. Now, to accommodate the books long housed in the original 105-year-old stacks, a part of the library whose future is still under discussion, the second floor is being turned into a state-of-the-art storage hub.

This was not, of course, Plan A. That plan entailed a makeover of the flagship Fifth Avenue library that would have sent the research books to Princeton, N.J. But it set off a virtual Fahrenheit 451 of outrage among scholars and others for whom the library’s role as a research mecca seemed endangered. Critics, who hoped the old steel stacks could stay in use, remain apprehensive about the new stocking and retrieval system, which they say is impressive but has not been tested.

“I applaud their efforts, and I want to be optimistic about the future,” said David Nasaw, an author, historian and professor at the City University of New York, and one of a number of critics who have recently met with library officials to discuss the site. “I still want to know how many books were taken out of the stacks and how many are coming back.”

Library officials say they hope to show they have moved past the quarrel over where to house the materials and are getting on with the business of making them safe and accessible.

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Behind the library, under Bryant Park and the skating rink, a complex storage space is taking shape.CreditJames Estrin/The New York Times

“We heard the concerns about our prior renovation plan,” said Anthony W. Marx, the library’s president. “Now, actions speak loudly, as we are here literally putting a stake in the ground, or more accurately stacks, to store and preserve the amazing collection on site for the research community.”

The nuts and bolts of the expansion are in many ways as riveting as the rare volumes and archival treasures that will line it, among them a handwritten 1630 land deed signed by the Dutch colonial governor Peter Minuit, and George Washington’s 1757 military recipe for “small beer,” the craft brew of his day.

The climate-controlled repository encompasses more than 110,000 square feet divided between the two stories. It stretches toward Sixth Avenue from beneath the back wall of the main building, which fronts Fifth Avenue, extending under a large part of the park between 42nd Street and 40th Street.

To complete the second floor, engineers used a hatch that opens onto Bryant Park to funnel in the tons of concrete needed to finish the floor. Because moisture is a dire threat to the books, builders surrounded the space with an extensive drainage network.

Soon, just below where skaters sip cocoa, a nerve center of librarians, curators and clerks, working at computer terminals in a constant 65-degree environment (with 40 percent humidity), will receive electronic requests for the research books and other items.

The retrieval system aims to get the materials from shelf to scholar in less than 40 minutes.

The library trustee Abby S. Milstein and her husband, Howard P. Milstein, donated $8 million toward the cost, and the storage space will be named the Milstein Research Stacks.

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An illustration shows the addition of library stacks beneath the surface of the Bryant Park lawn.CreditNYPL

When the expanded space opens, its upper and lower floors will be configured differently. While roughly half the upper floor has space for books, a large portion is used to store special collections, microfilm and archives. The bottom floor will house only books.

Both floors, though, will use the new retrieval system, which is replacing a rickety conveyor belt installed in the 1980s. The new two-track delivery system will allow the motor-driven carts to run up or down to their destinations, loaded with books and programmed to stop as designated.

“What’s nice now is that if a cart were to die, you simply remove the cart, rather than shutting down the whole system,” said Gerry Oliva, director of facilities management.

But new technology is not always the answer. Mr. Oliva said the Bryant Park storage space has long had mobile stacks that can be pushed tightly together to maximize space.

But on the top floor, the mobile units are opened and closed with a card-swipe and laser system. It is intended, in part, to keep them from closing accidentally, in horror-movie style, as clerks seek books.

That never happened, but on the new bottom floor, officials are reverting to good, old-fashioned hand cranks to maneuver the shelves.

Why? Excessive maintenance.

“The other ones just don’t work as well,” Mr. Oliva said.

Correction:

An article on Monday about the New York Public Library’s storage space for books underneath Bryant Park misstated the extent of the storage area and related spaces that serve it. While they occupy a large part of the area beneath the park, they do not extend west to Sixth Avenue. The article also misstated the circumstances under which the scholar David Nasaw, a critic of a plan now scrapped to store books in New Jersey, met with library officials. He met with them at the library to discuss the subterranean storage area; he did not tour the site with library officials.